Since energy issues and global environmental issues are becoming more serious, solar cells are receiving more attention as an alternative energy source for replacing fossil fuels. In the solar cell, carriers (electrons and holes) generated by light irradiation to a photoelectric conversion section composed of a semiconductor junction or the like are extracted to an external circuit to generate electricity. A collecting electrode is provided on the photoelectric conversion section of the solar cell for efficiently extracting carriers generated at the photoelectric conversion section to the external circuit.
For example, in a crystalline silicon-based solar cell using a single-crystalline silicon substrate or a polycrystalline silicon substrate, a collecting electrode made of a slender metal is provided on a light receiving surface. Also, in a heterojunction solar cell having amorphous silicon layers and transparent electrode layers on a crystalline silicon substrate, collecting electrode(s) are provided on the transparent electrode layer(s).
The collecting electrode of the solar cell is generally formed by pattern-printing a silver paste by a screen printing method. This method is simple in terms of the process itself, but has such a problem that the material cost of silver is high, and that the resistivity of the collecting electrode increases because a silver paste material containing a resin is used. For decreasing the resistivity of the collecting electrode formed of a silver paste, it is necessary to thickly print the silver paste. However, since the line width of the electrode increases with the increase of the print thickness, thinning of the electrode is difficult, and the shading loss by the collecting electrode increases.
For solving these problems, a method is known in which a collecting electrode is formed by a plating method excellent in terms of material and process costs. For example, Patent Documents 1 to 3 disclose a solar cell in which a metallic layer made of copper or the like is formed by a plating method on a transparent electrode that forms a photoelectric conversion section. In Patent Documents 1 and 2, first, a resist material layer (insulating layer) having an opening section matching the shape of a collecting electrode is formed on the transparent electrode layer of the photoelectric conversion section, and then a metallic layer is formed at the resist opening section of the transparent electrode layer by electroplating. Thereafter, the resist is removed to form a collecting electrode having a predetermined shape.
Patent Document 3 discloses a method in which an insulating layer of SiO2 or the like is provided on a transparent electrode, a groove extending through the insulating layer is then provided to expose the surface or side surface of the transparent electrode layer, and a metal collecting electrode is formed so as to be in conduction with an exposed area of the transparent electrode. Specifically, a method is proposed in which a metal seed is formed on the exposed area of the transparent electrode layer by a light induced plating method or the like, and a metal electrode is formed by electroplating with the metal seed as an origination point. This method is more advantageous in terms of material costs and process costs because it is not necessary to use a resist unlike Patent Documents 1 and 2. By providing a low-resistance metal seed, the contact resistance between a transparent electrode layer and a collecting electrode can be lowered.
In formation of a photoelectric conversion section of a solar cell, thin-films such as a semiconductor layer, a transparent electrode layer and a metal electrode are generally formed on the surface of a substrate by a plasma-enhanced CVD method, a sputtering method or the like. These thin-films exist not only on the front surface of a substrate, but also on the side surface and on the back surface since the deposited films wrap around thereon, and thereby cause a short circuit and leakage between the front surface and the back surface. For preventing the above-described wraparound, for example, a method is proposed in Patent Document 4 in which a semiconductor layer and a transparent electrode layer are formed with the peripheral end portion of a crystalline silicon substrate covered with a mask.
Patent Documents 5 and 6 disclose methods for preventing a short circuit by performing predetermined processing after forming a semiconductor thin-film and an electrode on a substrate. Specifically, Patent Document 5 discloses a method in which a groove is formed by laser irradiation, and thereafter a crystalline silicon substrate is cleaved along the groove to form a solar cell in which the side surface of a photoelectric conversion section is composed of a cut surface. In Patent Document 6, a method is proposed in which a semiconductor layer and a transparent electrode layer formed on a crystalline silicon substrate are removed by laser irradiation to form a groove. Either a semiconductor thin-film or an electrode is not present on the cut surface in Patent Document 5 and the surface of the groove in Patent Document 6, and therefore the problem of short circuit resulting from wraparound is solved.
In Patent Document 6, a configuration is illustrated in which a transparent electrode layer and a semiconductor layer of a conductivity type are removed by laser irradiation, but it is difficult to selectively remove only these layers by laser irradiation. Therefore, generally a groove formed by laser irradiation reaches the surface or the inside of a crystalline silicon substrate.